Conventional beverage dispensers such as used in restaurants and fast food facilities use a flow rate approach to dispense the liquid. Typically such dispensers use a timer to control the length of time that the fluid is permitted to flow into a cup or other receptacle of known capacity. For dispensers used with more than one size cup, button switches labelled "small", "medium" and "large" are provided to be actuated by the server to set the timer to the flow period to fill the particular size cup. Various problems arise with these mechanisms. When the pressure decreases, such as when the carbonating CO.sub.2 tank runs low, the flow also decreases, causing less than the correct amount to be delivered. When these timers are set for each cup size there is taken into account the typical amount of ice, if any, to be used. If the volume of ice varies from the typical amount, the cup will overflow or underfill, either wasting the beverage or requiring a manual override to complete the filling. These dispensers must run slowly enough to avoid excess foam. If foaming does occur there may be wasteful overflow, delays in service, and the need for one or more cycles of manual override to properly top-up the cup. Ever present is the simple problem of the server hitting the wrong size indicator button, which causes underflow or overfill with consequent delays, waste and wet, messy cups for the customer.
Attempts to more fully automate these dispensers, such as by using photoelectric devices, do not compensate for ice level, foam or flow rate, but such devices can be made to determine the size of the cup to be filled. However, photoelectric devices do suffer from alignment problems and environmental contamination. Mechanical probes have similar problems regarding distinguishing between different size cups and foam buildup. Mechanical probes also tend to be broken, interfere with the insertion and removal of the cup, and are difficult to keep clean. Capacitive proximity devices suffer from similar problems and the sensor portion must be properly located for each different size cup. Weighing devices also must know what size cup is placed on them and must distinguish between different amounts of ice initially in the cup, and are susceptible to errors due to jostling and vibrations.